Field of the Invention
This invention relates to solar energy absorbing surfaces, and in particular to selective absorbers having high solar thermal conversion efficiency and low heat loss after collection. One of the most common and effective methods of utilizing solar energy is the process which converts the sun's radiation into heat by having it impinge on an absorbing surface. This particular invention concerns a new type of absorbing surface, which is particularly suited for use when the solar thermal energy collection is in vacuum at temperatures above 100.degree. or 150.degree. C. It also is suitable for lower temperature operation.
Solar energy is a diffuse energy source, such that, even on bright days, only about one kilowatt of energy is incident on a square meter of surface area facing the sun. Thus, rather large areas are required to collect sufficient energy for most applications. To compete favorably with other sources of energy, the cost per unit area must be low.
An important factor affecting both cost and performance of a solar thermal energy collector is its efficiency. High thermal conversion efficiency, and thus lower area and cost, is obtained by using an absorbing surface which is black to the sun's radiation. Solar radiation covers a range of wavelengths from about 0.3 to 2.0 microns.
Competing with this requirement of high solar thermal conversion efficiency is the requirement of low heat loss after collection. Since the operating temperature of most solar thermal collectors does not exceed a few hundred degrees Celsius, it is possible to develop a solar absorbing surface, which is also a low emittance surface, for the infrared radiation emitted at the operating temperature of the absorbing surface. The wavelength range for this radiation is largely above a few microns. Such an absorbing surface, which is absorptive to the sun, but reflective to infrared radiation, and thus has low infrared emittance, is called a selective absorber.
For an absorbing surface operating in vacuum, the only heat loss within the vacuum is due to infrared radiation. At operating temperatures above 100.degree. or 150.degree. C., this radiative loss can be very large, unless the surface emittance is made low. One of the exceedingly difficult tasks in the formation of a selective absorber, is to make the absorptance for solar radiation sufficiently high, while the infrared emittance is made quite low. The emittance apparently always increases, for a particular process of absorber formation, as the absorptance is increased. The selective absorber, whose invention is disclosed here, can be made to have particularly favorable values of solar absorption and infrared emission. Fixed (non-tracking) solar thermal energy collectors operating at elevated temperatures, of the order of 200.degree. C. or above, must use vacuum and concentration, as well as high absorptance and low emittance for their absorbing surfaces, to achieve high collection efficiency. Collectors of this type have stagnation (no heat removal) temperatures which can exceed 400.degree. or even 500.degree. C. Such stagnation temperatures would occur, if pumping of the heat transfer fluid through the collector is interrupted for a sufficient length of time.
Numerous selective absorbers have been developed and used in the prior art for solar energy. Selective absorbers on metal substrates are most commonly obtained by electrodeposition of black chrome or black nickel on an infrared reflecting sublayer or substrate, or by the conversion of a metal coating or surface to a metal oxide on an infrared reflecting layer or substrate. Representative examples are found in U.S. Patents by Tabor, U.S. Pat. No. 2,917,817; Lowery, U.S. Pat. No. 3,920,413; McDonald, U.S. Pat. No. 4,055,707; and Roberts, et al, U.S. Pat. No. 4,104,134. Black chrome, apparently the best of these coatings for temperature operation below about 300.degree. C., is a relatively expensive electroplated coating. Black chrome and black nickel are not stable at temperatures of the order of 350.degree. C. or above in vacuum. Black nickel is not stable in air at even lower temperatures. The metal oxides are usually not stable at similar temperatures in vacuum, and may change their properties when heated in air. These oxide selective absorbers often have less desirable optical properties.
As currently known, the selective absorbers used for evacuated collectors are mostly vacuum evaporated or sputtered, usually onto a glass substrate. The details of forming vacuum evaporated interference coatings has been known for approximately twenty years.
The equipment for vacuum evaporation and sputtering is expensive and does not lend itself to mass production of selective absorbers. At this time, production consists of batch processing, and is limited to solar collector tube lengths of the order of one meter. Use of such short tube lengths greatly increases the manifold losses of an array of these collector tubes.
The formation of solid carbon from carbonaceous gases has been studied and used extensively. Such formation, when performed at elevated temperatures, by the decomposition of carbon containing gaseous compounds is termed pyrolytic carbon.
There are numerous U.S. Patents involving pyrolytic deposition of carbon on solid surfaces. Examples include Hobrock, U.S. Pat. No. 2,057,431 and Palumbo, U.S. Pat. No. 2,487,581, for making carbon resistors, Bokros, et al, U.S. Pat. No. 3,298,921, Goeddel, et al, U.S. Pat. No. 3,619,241, and Adams, et al, U.S. Pat. No. 4,194,027, for coating nuclear reactor fuel particles; Froberg, U.S. Pat. No. 3,944,686 for coating pyrolytic carbon on porous sheets of carbon material; and Batchelor, U.S. Pat. No. 3,317,338 for coating carbon on graphite rocket nozzles.
Nothing in the prior art teaches the pyrolytic deposition of carbon to form a selective absorber. Further, the carbon coatings formed for these other purposes are generally much thicker and of a different structure than those required for selective absorber formation, and these coatings are formed at a much higher temperatures than those generally contemplated for the invention disclosed here.
When studies of carbon formation on catalysts are made, invariably, the motivation is to understand the process and detect what is occurring, in order to learn how to eliminate or reduce the poisoning (reduced activity) of the catalyst caused by the carbonaceous layer. The carbonaceous layers and substrates would generally be completely unsuitable for use as selective absorbers. No efforts have been directed toward developing a thin carbonaceous coating on a metal catalyst for any purpose, prior to this invention.
A carbon coating for solar collectors has been developed by Googin, et al, U.S. Pat. No. 4,048,980, where carbon is mixed with a binder and painted or sprayed on a metal substrate for this coating. It has high emittance and would be completely unsuitable for use at higher operating temperatures. Peterson, in U.S. Pat. No. 4,065,593, teaches the use of carbon fibers for the absorption of solar radiation. These fibers do not exhibit selective absorption and are formed by an entirely different process than that used here.
Spielberg, in U.S. Pat. No. 3,968,786, teaches the use of a conductive carbonaceous pyropolymer for the absorption of solar radiation.